The need for storage of water is well known, especially in the west where many regions are arid or semi-arid. Water may be stored for a variety of reasons: potable water for household use; water for livestock; and fire cisterns are some of the more common applications. While open, above ground storage is common for livestock, enclosed storage is preferred for potable water in order to avoid contamination. Where enclosed tanks are used, underground tanks are often preferred. By burying the tank, the tank itself is supported, the tank is protected, and the contents of the tank are insulated from temperature changes. Underground tanks are also commonly used as septic tanks where municipal sewer service is not available.
Various materials have been used to construct underground storage tanks. Steel and concrete have been in use for decades. Fiberglass is a newer material rapidly gaining popularity especially in petroleum storage. Steel tanks are prone to rusting, especially where they are exposed to ground water. Concrete tanks do not rust, but are semi-porous and will deteriorate with time. Fiberglass has good resistance to corrosion, but is relatively brittle, requiring careful handling, especially during installation. A sharp blow or inadvertent contact with the installation equipment can easily damage a fiberglass tank.
Both steel and concrete tanks are relatively heavy. This typically results in the tanks being constructed relatively near the point of installation to reduce transportation difficulties and expense. This weight also effectively limits the maximum size of tank which can be constructed of these materials. Fiberglass is a much lighter material and can be used to fabricate a tank which is relatively rigid for its size. This enables the construction of relatively large, light weight tanks which are efficient to build and transport. However, there must be sufficient room for the necessary equipment, such as a crane, at the job site to off load and install the tank. Additionally, construction of fiberglass tanks is a labor intensive process which makes them relatively expensive.
Polyethylene has several characteristics which make it particularly suitable for potable water storage. It is typically formed into tanks using a rotary molding process which results in a one piece, seamless tank. This limits opportunities for bacterial growth and ground water infiltration. The material is impact resistant, flexing rather than cracking like fiberglass, and is highly corrosion resistant. However, this flexibility results in a vessel which is structurally weaker, limiting the size of the tank which can be cost effectively constructed. Large radiuses and flat surfaces are prone to buckling and collapse when the surrounding ground shifts or freezes, or from external pressure due to ground water.
Where access to the site is restricted, none of the above approaches is ideal. This is a common situation in mountain communities where the terrain limits access. Roads are often narrow and sharply curved. Surfaces may be rough and unimproved, with dirt or gravel being common. However, houses and ranches in these regions are often the ones most in need of water storage because municipal water is unavailable, natural water supplies may not exist, and wells are difficult and expensive to drill and may provide only a minimal flow. In such a situation, a fire cistern may be needed for safety and is often a prerequisite to obtaining fire insurance.
Compounding the problem is the fact that the required storage capacity often exceeds the capacity of any single tank which can be efficiently moved to the site and installed. It thus becomes necessary to interconnect several tanks to form a storage system of sufficient capacity. Several designs for such systems are known. Most common are tanks which couple together with mating pipe flanges. Where the terrain is uneven, achieving the required alignment to allow insertion of all of the bolts which join the flange can be very difficult. If it is achieved, shifting of the tanks due to settling can result in significant stresses in the tanks. The strict alignment requirements can increase the cost of the excavation as it must be matched to the tank, there being little adaptability in the tank. The level of skill required to properly install and join these types of tanks is often not available in remote areas, requiring bringing in skilled labor for the installation. Further, supplies or materials may be required which are not commonly available. If additional items are needed, the project may be delayed while they are brought in.
Where the tank material is polyethylene, manufacturing characteristics of the material introduce additional problems. Uneven cooling of the mold and relative time of release of various portions of the tank from the mold cavity cause warping and distortion in the tank which can vary from one unit to another. While not significant to the performance of the tank, these variations can easily be large enough to make it difficult to align all of the bolts for an interconnecting flange, and nearly impossible to align multiple connections on the face of a tank.
One solution to the alignment problem is to interconnect the tanks with flexible hose or tubing. This allows for misalignment between the tanks and eliminates the flange mating problem. However, the joints used for this type of connection are prone to leaking and require periodic inspection and repair, which typically require at least partial excavation. Additionally, the hardware components of the joint are external to the tank and are exposed to the environment and backfill material both during and after installation.
There is a need for a relatively small, lightweight, modular tank which can be transported to the installation site over narrow, unimproved roads using light to medium duty equipment. At the site, it should be possible to easily unload and quickly connect several tanks together to provide a larger storage system. Ideally, the system should provide various configurations which are adaptable to the terrain or needs of the user. Alignment between the various modules must not be critical. It should be possible to install and connect the modules using unskilled labor and materials which are commonly available in rural areas. Ideally, it should be possible to periodically inspect, and if necessary repair, the inter-tank connections after installation of the system without excavation.